This invention relates to a high dispersion optical glass of a SiO.sub.2 -Nb.sub.2 O.sub.5 -R'.sub.2 O (where R' is an alkali metal element)-F system having optical constants of a refractive index (Nd) of about 1.52-1.67 and an Abbe number (.upsilon. d) of about 32-50 which is excellent in light transmissivity and contains a relatively large content of Nb.sub.2 O.sub.5 ingredient.
Known in the art as optical glasses having the above described optical constants are glasses of a SiO.sub.2 -PbO-R'.sub.2 O system, a SiO.sub.2 -TiO.sub.2 -R'.sub.2 O system and a SiO.sub.2 -TiO.sub.2 -R'.sub.2 O-F system disclosed in the specification of U.S. Pat. No. 2,481,701. The first mentioned prior art optical glass, however, contains toxic PbO and the latter two optical glasses are disadvantageous in that they have not excellent light transmissivity.
Japanese Patent Application Laid-open No. Sho 52-25812 discloses a glass of a SiO.sub.2 -TiO.sub.2 -Nb.sub.2 O.sub.5 -R'.sub.2 O system and Japanese Patent Application Laid-open No. Sho 52-45612 discloses a glass of a SiO.sub.2 -Nb.sub.2 O.sub.5 -R".sub.2 O (where R" is a divalent metal element)-R'.sub.2 O system. These glasses, however, do not sufficiently satisfy both a high dispersion optical characteristic and an excellent optical transmissivity which are the target of the present invention.
It is, therefore, an object of the invention to eliminate the disadvantages of the prior art optical glasses comprehensively by providing a novel optical glass which is free from the toxic PbO ingredient, has optical constants of a refractive index (Nd) of about 1.52-1.67 and an Abbe number (.upsilon. d) of about 32-50, and has an excellent light transmissivity.